The present invention relates generally to improvements in key-exchangeable locks and, it relates more particularly to an improved lock mechanism permitting the reliable performance of a key-exchanging procedure without any improper operation during the course of such procedure.
With the key-exchangeable lock of the prior art, an error in the sequence of the key-exchanging procedure not only makes it impossible to exchange the existing key to a desired new key but also results in the disabling of the releasing mechanism and it has usually been necessary to then disassemble the lock and to reassemble the tumblers. Such a lock has been disclosed, for example, by U.S. Pat. No. 4,072,032. In this prior art lock, the centre around which the tumblers are rotated is displaceable and, as a consequence, the running plate is also movable even when the predetermined key does not assume the lock releasing position in which the tumblers are held at their released positions. As a result, an erroneous operation would bring the fence out of the gates of the tumblers so that the tumblers are disassembled. If the operator is not familiar with the proper sequence of the key-exchanging procedure, there is a danger that such an erroneous operation might often occur.